Decades after the introduction of intensity modulated radiotherapy to the clinic, precise dosimetric commissioning of complex radiotherapy treatments remains a clinical challenge. With the advent of commercially available magnetic resonance imaging guided radiotherapy (MR-IGRT) systems, the permanent magnetic field introduces yet another source of uncertainty in the prediction of dose distributions—an already error-prone process.
Image guidance in high-precision radiotherapy affords the ability to frequently image during a treatment to improve accuracy of the dose delivery (i.e. gating, adaptive planning). In conjunction with delivery systems (such as a linear accelerator for photons and electrons, or cyclotron for protons), magnetic resonance imaging (MRI), computed tomography (CT) and x-ray imaging help visualize anatomical structures prior to or during treatment, especially movement-prone regions (i.e. lungs, liver and prostate). As the complexity of these devices increases so do the clinical needs to maintain a high-standard of care. The MRIdian® (by ViewRay) is a novel example of cutting-edge MR-IGRT technology which could benefit from true multi-dimensional dose validation.
ViewRay's MRIdian® system affords real-time MR image guidance during treatment, with enhanced soft-tissue contrast over on-board CT at no additional dose of radiation to the patient. However, the presence of an MR-field for on-board imaging in radiotherapy introduces clinical hurdles. Two examples include (1) asymmetry of the point spread kernel in homogeneous tissue, leading to reduced build-up and asymmetric penumbra, and (2) the electron return effect (ERE), which occurs at the boundaries between tissue and air.
To combat these hurdles, recent studies have tested the feasibility of in-situ 3D dosimetry with three different gels (BANG, MAGIC and Fricke) on the ViewRay. Similar works provide a dosimetry program for ViewRay's patient-specific IMRT QA, which include one-dimensional multipoint ionization chamber measurement, two-dimensional (2D) radiographic film measurement using a 30×30×20 cm phantom with multiple inserted ionization chambers, quasi-three-dimensional diode array (ArcCHECK) measurement with a centrally inserted ionization chamber, 2D fluence verification using machine delivery log files, and 3D Monte-Carlo Simulation.
In 2013, Juang, et al., investigated the feasibility of remote high-resolution 3D dosimetry with the PRESAGE®/Optical-CT system. Measured on-site (t=1 hr) and remote (t=48 hr) scans were taken of simple 4-field box deliveries and served as the reference distribution for 3D gamma analysis. Both on-site and remote measurements agreed well with TPS dose, with passing rates of 97.4%±2.2% and 97.6%±0.6%, respectively, for 3%/3 mm, criteria 10% dose threshold. However, some discrepancies were observed, for which corrections would need to be derived to achieve clinical grade dosimetry. In this study, changes in optical density between on-site and remote scanning in small volume dosimeters were observed.
In other past studies, volume and temporal effects have been noted where sensitivity and response differs over time and throughout the dosimeter. Thus, there remains a need to provide a comprehensive method that corrects for these effects.